The present invention relates to a process for producing pregelatinized potato starch (hereinafter referred to as alpha-potato starch) for use as a binder in feeds for fish, particularly, feeds for eel.
In cultivation of fish, for example, eel, alpha-potato starch is commonly incorporated in feeds as a binder. Because of its transparency and higher viscosity, alpha-potato starch is best suited for use as a binder in feeds for eel. However, the potato-starch which is to be thermal gelatinized is a natural product and its quality differs greatly from one lot to another. Furthermore, the quality of the potato-starch has direct effects on the final quality of the alpha-potato starch. As a matter of fact, manufacturers of alpha-potato starch have long found it necessary to maintain the quality of their products by using select and therefore costly starch.
The conventional feed for cultivating eel is based on white fishmeal which is supplemented with vitamins or nutrients plus 20-25% of alpha-potato starch as a binder. The eel grower adds 5-10% feed oil and 110-130% water to the feed, kneads them with a mixer, and feeds the eel with the resulting mixture. The alpha-potato starch is required to satisfy the following conditions: (1) it keeps the integrity of the feed in water until the eel has finished its eating; (2) it is easily digestable and retains the viscoelasticity of the feed that assits in the eating by the eel; (3) it inhibits the dissolution of water-soluble additives; (4) it ensures easy removal of the feed from the mixer. The quality of alpha-potato starch largely depends on the quality of white fishmeal, the temperature of water in which the eel is grown, and the temperature of atmosphere. Therefore, actual alpha-starch products have various viscosity characteristics so adjusted as to provide the proper viscoelasticity according to the quality of the fishmeal and the seasonal change. As already mentioned, alpha-starch producers attain this object by using select potato starch. In addition, they control the operating conditions of a conventionally used drum dryer, particularly, the heating temperature and the rotational speed of the drum. However, the change in the operating conditions of the drum could cause a significant drop in the efficiency of the dryer and is not desired.
As is well known, the physical properties of gelatinized potato starch vary greatly according to the content of phosphoric acid that forms an ester with the glucose of which the starch is made, as well as to the composition of metallic ions bonded to that phosphoric acid. Phosphoric acid is present in the potato tissue in the form of a potassium salt, but during the starch production, the potassium ion is replaced by divalent calcium and/or magnesium ion in the process water, which reduces the gelatinizing properties of the starch and adversely affects the quality of alpha-starch. The starch slurry also contains free ions that are not bound to the phosphoric acid. Extensive studies have been made on the effects of various free ions on the viscosity characteristics of the starch, but nobody has studied their effects on the quality of alpha-starch.